1. Field of the Invention
The present invention relates to an automatic control type electronic flash apparatus which is capable of automatically controlling the quantity of light emitted and, more particularly, to a device for indicating whether or not the quantity of light emitted provides an over-exposure.
In an automatic control type electronic flash unit of the type described above, flashlight emitted by the flash unit and reflected by an object to be photographed is received by a light-receiving element. When the quantity of light received reaches a predetermined value which provides a proper exposure, the light-receiving element generates an emission stop signal to stop the flash operation of the unit. FIG. 1 schematically shows the strength of flashlight emitted by the flash unit toward the object as a function of emission time. The ratio of the quantity of light received by the light-receiving element to the quantity of light emitted by the flash unit decreases with an increase in the distance between a camera and the object to be photographed. Therefore, when the object is at a relatively great distance from the camera, a great quantity of light must be emitted to provide a proper exposure. As may be seen from FIG. 1, the emission stop signal is generated at time .tau.1 after a considerable time period from flash emission start time .tau.0 so as to interrupt the emission of flashlight. Thus, a relatively great quantity of light represented by an area S1 is emitted. On the other hand, when the object is near the camera, the above-mentioned ratio increases and the quantity of light which provides a proper exposure decreases. For example, when the object is very close to the camera, the emission stop signal is generated at time .tau.2 immediately after time .tau.0, and the quantity of light emitted up to this point corresponds to an area S2 which is very small. There is a predetermined delay time .DELTA..tau. between generation of the emission stop signal and interruption of the flashlight emission. The flash unit emits excess quantities of light .DELTA.S1 and .DELTA.S2 during the predetermined delay time .DELTA..tau.. The excess quantity of light .DELTA.S1 emitted when the object is at a relatively great distance from the camera is sufficiently smaller than the relatively great quantity of light corresponding to the area S1 to be negligible. For this reason, a proper exposure may be obtained despite the excess quantity of light .DELTA.S1. In contrast to this, the excess quantity of light .DELTA.S2 emitted when the object is very close to the camera is not negligible in comparison with the relatively small quantity of light corresponding to the area S2 and provides an over-exposure. Two methods for detecting an over-exposure in flash photography of a near object are disclosed in U.S. Pat. No. 4,122,465 assigned to Nippon Kogaku K.K. According to the first method disclosed in this patent, an over-exposure is indicated if an emission stop signal is generated within a predetermined time from flash emission start (or triggering for light emission). According to this method, since the charged voltage in a main capacitor for flash operation is not always constant, the amount of flashlight during the predetermined time does not become constant so that reliable indication of an over-exposure may not be guaranteed. According to the second method, the quantity of light reflected by the object to be photographed is received and a current corresponding thereto is integrated. When the integrated current exceeds a predetermined value, an over-exposure is indicated. According to this method, the detection precision is not affected by the charged voltage of the main capacitor, as in the first system, so that a more reliable indication of an over-exposure may be performed. However, the voltage integrated by an integrating capacitor is an analog voltage which is easily subject to noise in the transmission system. For this reason, the detecting circuit must be arranged near the integrating capacitor. This limits the circuit design significantly. Furthermore, when metering circuits are arranged in the flash unit and the camera, respectively and, TTL control is performed (whereby light transmitted through the photo-taking lens is measured and the quantity of light emitted by the flash unit is controlled according to the measurement) using the metering circuit of the camera as needed or when the metering circuit of the flash unit is used, the detecting circuit must be incorporated for both the camera and the flash unit since it must be close to the integrating capacitor.